6351 sampling integration time vs. rate

[dstanisl]

I would like to know how the 6351 and other X-series DAQs sample. Specifically, given a sampling rate of v and a sampling period of p, the integration time of each sample must be less than p. My question is, during the sampling period, what is actually going on? Is it integrating over all of p? Is there some settling time? Does the integration time even depend on p? Are there any settings to affect the way the DAQ uses the sampling period?

Essentially what I'm looking for is a full description of what goes on during p, something like "p/3 settling time, 2p/3 integration time".

[Seth_B. · Accepted solution]

So, there are two relevant "rates" when sampling on a multiplexed board like the NI 6351.  There is the "Sample Rate" which is the rate at which a sample for each channel is taken and then there is the "Convert Rate" which is the rate at which each channel is connected to the multiplexer in order to take a single sample.  I've tried to show this below
:

Each time a sample needs to be taken, each channel needs to be connected to the ADC in turn.  The Convert Clock controls how fast this happens.

The convert rate is dependant on the sample rate.  If you choose a sample rate that is slow enough, we will wait 10uS between converting each channel to allow for more settling time.  If you rate is too fast for the 10uS wait between each channel, we convert as fast as necessary to achieve you sampling rate.  At max rate, the board is cycling through each channel as fast as possible in order to keep up.

Within the convert period, we do a number of things.  We switch the multiplexer to the channel, wait a short period of time to settle (this time is what varies based on your sampling rate), then we perform the conversion with the ADC (this is what you'd call "integration time").  The switching time and conversion time are not user controlled; only the settling time varies based on your sample rate and convert clock rate settings.

There isn't a forumla that I can give you for how long each of these parts takes.  The best you can do is read the DAQmx Timing property "AI Convert Rate".  This will tell you the total rate for switching, settling, and converting.

What do you need to know the integration time for?

[dstanisl]

I'm running an application where the AO channel of my 6351 is connected to an amplifier whose output is connected to a device. We sweep the voltage across the device and measure the voltage both across the device and across a small current sense resistor. With this information we can generate a nice I-V curve.

The problems start when you consider the power requirements of this measurement and the limits of the op-amp we use. We must perform the sweep within 5ms, so I have the DAQ running at full speed. In this case the integration time and other properties of the sampling are important. If I'm running three AI channels at 400kHz, there are 2.5us to sample them. I've already had to add a follower to part of the circuit to eliminate ghosting.

The reason I'm asking is to get a better understanding. At the moment there is no specific problem I need to solve, but much room for improvement.

Thanks for your very detailed response. It's exactly what I was looking for.